Attachment changing page's permalink - $post not resetting?

Question

I have a function that I use to populate from checkboxes on a page's edit view (post.php?action=edit). I call this function when I render a metabox on the editor page.

function get_available_attachments() {
    global $post;

    $attachments = [];
    $attach_query = new WP_Query([
        'post_type' => 'attachment',
        'post_status' => 'inherit', // default is 'publish'
        'post_mime_type' => 'application/pdf',
        'orderby' => 'name',
        // 'orderby' => 'date',
        'order' => 'DESC',
    ]);

    while($attach_query->have_posts()) {
        $attach_query->the_post();
        $attachments[] = (object)[
            'mime_type' => $post->post_mime_type,
            'title' => $post->post_title,
            'ID' => $post->ID
        ];
    }

    wp_reset_postdata();

    return $attachments;
}

For some reason, when I save/update the post, the permalink is changed to having the slug of an attachment! I thought wp_reset_postdata(); would stop the attachment from interfering, but I have no idea what's going on. In the save_post action that I hook into, I only receive numerical IDs of some attachments, I don't call that function again.

EDIT for more info If I echo the post slug, this is the behaviour:

function get_available_attachments() {
    global $post;
    echo $post->post_name .'<br>'; // correctly page slug

    $attachments = [];
    $attach_query = new WP_Query([
    // ...

    echo $post->post_name .'<br>'; // correctly an attachment slug
    wp_reset_postdata();
    echo $post->post_name .'<br>'; // INCORRECT still attachment slug

Somehow the wp_reset_postdata(); doesn't work

Answer 1

I call it when I render a metabox on the edit page. (

Yeah, you shouldn't be using globals in this context - use a foreach loop instead:

function get_available_attachments( $post /* This is the post object being edited */ ) {
    $attachments = [];
    $attach_query = new WP_Query([
        'post_type'      => 'attachment',
        'post_status'    => 'inherit', // default is 'publish'
        'post_mime_type' => 'application/pdf',
        'orderby'        => 'name',
        // 'orderby'     => 'date',
        'order'          => 'DESC',
    ]);

    foreach ( $attach_query->posts as $att ) {
        $attachments[] = ( object ) [
            'mime_type' => $att->post_mime_type,
            'title'     => $att->post_title,
            'ID'        => $att->ID
        ];

    }

    return $attachments;
}